U.S. patent number 6,252,512 [Application Number 09/263,038] was granted by the patent office on 2001-06-26 for monitoring system and method.
This patent grant is currently assigned to Hill-Rom, Inc.. Invention is credited to Carl William Riley.
United States Patent |
6,252,512 |
Riley |
June 26, 2001 |
Monitoring system and method
Abstract
A system including piezoelectric material wherein a force
exerted on the piezoelectric material is determined by subjecting
the piezoelectric material to an input signal of sweeping frequency
and evaluating the response of the piezoelectric material to the
applied signal.
Inventors: |
Riley; Carl William (Milan,
IN) |
Assignee: |
Hill-Rom, Inc. (Batesville,
IN)
|
Family
ID: |
23000124 |
Appl.
No.: |
09/263,038 |
Filed: |
March 5, 1999 |
Current U.S.
Class: |
340/665; 340/666;
340/667; 73/862.045 |
Current CPC
Class: |
G01L
1/162 (20130101) |
Current International
Class: |
G01L
1/16 (20060101); G08B 021/00 () |
Field of
Search: |
;340/665-667,657,658,573.1,666 ;73/760,862.045,862.046
;345/173 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
60-222734 |
|
Nov 1985 |
|
JP |
|
96/31361 |
|
Oct 1996 |
|
WO |
|
Primary Examiner: Lieu; Julie
Attorney, Agent or Firm: Barnes & Thornburg
Claims
What is claimed is:
1. A monitoring system comprising:
a piezoelectric device which is subject to dynamic and static
forces whose magnitude is to be monitored and which generates a
signal for a change of force; and
a second device including a switch having a first state and a
second state for switching the second device between a first mode
corresponding to the first state and second mode corresponding to
the second state for detecting signals generated by the
piezoelectric device and determining the magnitude of force from
the detected signal for dynamic forces in a first mode, and
determining the impedance of the piezoelectric device and
determining the magnitude of force from the impedance for static
forces in a second mode.
2. The apparatus of claim 1, wherein the second device comprises a
device for providing electrical signals for exciting the
piezoelectric device and for determining the piezoelectric device's
response to the electrical signals to determine the impedance.
3. The apparatus of claim 1, comprising a plurality of said
piezoelectric devices coupled together in an array.
4. The apparatus of claim 3 wherein the piezoelectric devices are
formed into an array, the apparatus further comprising a third
device for holding the plurality of piezoelectric devices in the
array.
5. The apparatus of claim 1 wherein the second device comprises a
signal generator and an impedance determining circuit, switching of
the switch to the second state causing a signal to be applied by
the signal generator to the piezoelectric device and causing the
second device to determine the impedance of the piezoclectric
device to the applied signal.
6. The apparatus of claim 5, wherein the second device comprises a
device for determining from the determined force whether an object
is on the piezoelectric device.
7. The apparatus of claim 5 wherein the second device comprises a
device for sweeping the signal frequency and determining the
piezoelectric device impedance in response to the signal.
8. The apparatus of claim 1 wherein the second device comprises a
device for detecting a relatively large signal generated by the
piezoelectric device, placing the second device in the second mode
when the relatively large signal is detected, then determining if
the signal generated by the piezoelectric device varies at a
predetermined rate over a predetermined time, and then concluding
that an object is still exerting force on the piezoelectric
device.
9. The apparatus of claim 1 wherein the second device comprises a
device for detecting a relatively large signal generated by the
piezoelectric device, then placing the second device in the second
mode when the signal decreases over a brief time interval below
some minimum threshold, and then determining if a relatively
constant force is being applied.
10. The apparatus of claim 1 wherein the piezoelectric device
comprises a piezoelectric ceramic transducer.
11. The apparatus of claim 10 comprising an array of said
piezoelectric ceramic transducer.
12. The apparatus of claim 10 wherein the ceramic transducer is
custom fabricated to a particular size, shape, and electrical
properties.
13. The apparatus of claim 10 wherein the piezoelectric device is
oriented at a strategic load-bearing point.
14. The apparatus of claim 10 comprising a plurality of
piezoelectric devices coupled together and oriented at a plurality
of strategic load bearing points.
15. The apparatus of claim 1 wherein the second device comprises a
device for determining from the determined force whether an object
is exerting force on the piezoelectric device.
16. The apparatus of claim 1 wherein the second device comprises a
device for detecting a relatively large signal generated by the
piezoelectric device, placing the second device in the second mode
when the relatively large signal from the piezoelectric device is
detected, then determining if the signal generated by the
piezoelectric device varies at a predetermined rate over a
predetermined time, and then concluding that an object is exerting
force on the piezoelectric transducer.
17. The apparatus of claim 1 wherein the second device comprises a
device for detecting a relatively large signal generated by the
piezoelectric device, placing the second device in the second mode
when the signal decreases relatively rapidly to below a minimum
threshold, and then determining if a relatively constant force is
being applied on the piezoelectric device.
18. The apparatus of claim 1 wherein the second device comprises a
device for sweeping the signal frequency and determining the
piezoelectric device's impedance in response to the swept signal
frequency.
19. The apparatus of claim 1, wherein the piezoelectric comprises a
flexible piezoelectric film.
20. The apparatus of claim 19 wherein the flexible piezoelectric
film is custom fabricated to a particular size, shape, and
electrical properties.
21. The apparatus of claim 19 wherein the flexible piezoelectric
film comprises an array of said piezoelectric devices.
22. The apparatus of claim 1, wherein the piezoelectric device
comprises a coaxial cable including a center conductor, a shield,
and a piezoelectric material between the center conductor and
shield.
23. The apparatus of claim 1 wherein the second device comprises a
signal generator, the second device switching of the switch to the
second state causing a signal to be applied by the signal generator
to the piezoelectric device and causing the second device to
determine the impedance of the piezoelectric device to the applied
signal.
24. The apparatus of claim 23 wherein the second device comprises a
device for determining from the determined force whether an object
is exerting force on the piezoelectric device.
25. The apparatus of claim 1 wherein the second device comprises a
device for detecting a relatively large signal generated by the
piezoelectric device, placing the second device in the second mode
when the relatively large signal from the piezoelectric device is
detected, then determining if the signal generated by the
piezoelectric device varies at a predetermined rate over a
predetermined time, and then concluding that an object is exerting
force on the piezoelectric device.
26. The apparatus of claim 23 wherein the second device comprises a
device for sweeping the signal frequency and determining the
piezoelectric device's impedance in response to the swept signal
frequency.
27. A monitoring method comprising:
providing a piezoelectric device which is subject to dynamic and
static forces whose magnitude is to be monitored and which
generates a signal for a change of force;
detecting signals generated by the piezoelectric device for dynamic
forces;
determining the magnitude of force from the detected signal for
dynamic forces;
determining the impedance of the piezoelectric device for static
forces if the generated signals are not detected; and
determining the magnitude of force from the impedance for static
forces.
28. The method of claim 27 wherein determining the impedance of the
piezoelectric device comprises providing electrical signals for
exciting the piezoelectric device and determining the piezoclectric
transducer's response to the electrical signals.
29. The method of claim 27 wherein providing a piezoelectric device
comprises providing a plurality of said piezoelectric devices
coupled together in an array.
30. The method of claim 27 wherein detecting signals generated by
the piezoelectric device comprises providing a switch having a
first state corresponding to a first mode of operation when the
force applied to the piezoelectric device is being modulated and a
second state corresponding to a second mode of operation when the
force applied to the piezoelectric device is not being
modulated.
31. The method of claim 30, including detecting a relatively large
signal generated by the piezoelectric device, then determining if
the signal generated by the piezoelectric device varies at a
predetermined rate over a predetermined time, and then concluding
that an object is still on the piezoelectric device.
32. The method of claim 30, including detecting a relatively large
signal generated by the piezoelectric device, then detecting when
the signal decreases relatively rapidly to below a minimum
threshold, and then determining if a relatively constant force is
being applied.
33. The method of claim 30 wherein switching of the switch to the
second state causes a signal to be applied to the piezoelectric
device and causes the impedance of the piezoelectric device to the
applied signal to be determined.
34. The method of claim 33 further comprising determining from the
force whether an object is on the piezoelectric device.
35. The method of claim 33 wherein determining the impedance of the
piezoelectric device comprise sweeping the applied signal's
frequency and determining the piezoelectric device's impedance in
response to the swept signal frequency.
36. The method of claim 27 wherein providing a piezoelectric device
comprises providing a piezoelectric ceramic transducer.
37. The method of claim 36 wherein providing a piezoelectric
ceramic transducer comprises providing an array of piezoelectric
ceramic transducers.
38. The method of claim 37 wherein providing an array of said
piezoelectric devices coupled together further comprises providing
a third device for holding the plurality of piezoelectric devices
in the array.
39. The method of claim 36 wherein providing a piezoelectric
ceramic transducer comprises custom fabricating a piezoelectric
ceramic transducer to a particular size, shape, and electrical
properties.
40. The method of claim 36 further comprising orienting the
piezoelectric device at a strategic load-bearing point.
41. The method of claim 36 including providing a switch having a
first state when the force applied to the piezoelectric device is
being modulated and a second state when the force applied to the
piezoelectric device is not being modulated.
42. The method of claim 41 wherein determining the impedance of the
piezoelectric device's comprise providing electrical signals for
exciting the piezoelectric device and determining the impedance of
the piezoelectric device to the electrical signals.
43. The method of claim 42, including determining from the
determined force whether an object is exerting force on the
piezoelectric device.
44. The method of claim 42 including detecting a relatively large
signal generated by the piezoelectric device, then determining if
the signal generated by the piezoelectric device varies at a
predetermined rate over a predetermined time, and then concluding
that the object is exerting force on the piezoelectric device.
45. The method of claim 42 wherein providing electrical signals for
exciting the piezoelectric device and determining the impedance of
the piezoelectric transducer to the electrical signals comprise
sweeping the applied signal's frequency and determining the
piezoelectric transducer's impedance in response to the swept
signal frequency.
46. The method of claim 41 including detecting a relatively large
signal generated by the piezoelectric device, then detecting when
the signal decreases relatively rapidly to below a minimum
threshold, and then determining if a constant force is being
applied.
47. The method of claim 36 wherein providing a piezoelectric device
comprises providing a plurality of piezoelectric devices coupled
together and oriented at a plurality of strategic load-bearing
points.
48. The method of claim 27 wherein providing a piezoelectric device
comprises providing a flexible piezoelectric film.
49. The method of claim 48 wherein providing a flexible
piezoelectric film comprises custom fabricating a flexible
piezoelectric film to a particular size, shape, and electrical
properties.
50. The method of claim 48 wherein providing a flexible
piezoelectric film comprises providing an array of said
piezoelectric device.
51. The method of claim 48 including providing a switch having a
first state when the force applied to the piezoelectric device is
being modulated and a second state when the force applied to the
piezoelectric device is not being modulated.
52. The method of claim 51 including detecting a relatively large
signal generated by the piezoelectric device, then determining if
the signal generated by the piezoelectric device varies at a
predetermined rate over a predetermined time, and then concluding
that the object is exerting force on the piezoelectric device.
53. The method of claim 51 including providing electrical signals
for exciting the piezoelectric device and determining the impedance
of the piezoelectric device to the electrical signals.
54. The method of claim 53 including determining from the
determined force whether the object is exerting force on the
piezoelectric device.
55. The method of claim 53 wherein providing electrical signals for
exciting the piezoelectric device and determining the impedance of
the piezoelectric device to the electrical signals comprise
sweeping the applied signal's frequency and determining the
piezoelectric device's impedance in response to the swept signal
frequency.
56. The method of claim 51 including detecting a relatively large
signal generated by the piezoelectric device, then detecting when
the signal decreases relatively rapidly to below a minimum
threshold, and then determining if a constant force is being
applied.
57. The method of claim 27 wherein providing a piezoelectric
transducer comprises providing a coaxial cable including a center
conductor, a shield, and a piezoelectric material between the
center conductor and shield.
58. The method of claim 57 including providing a switch having a
first state when the force applied to the piezoelectric device is
being modulated and a second state when the force applied to the
piezoelectric device is not being modulated.
59. The method of claim 58 wherein determining the impedance of the
piezoelectric device comprise providing electrical signals for
exciting the piezoelectric device and determining the impedance of
the piezoelectric transducer to the electrical signals.
60. The method of claim 59, including determining from the
determined force whether the object is exerting force on the.
61. The method of claim 59 wherein providing electrical signals for
exciting the piezoelectric device and determining the impedance of
the piezoelectric device to the electrical signals comprise
sweeping the applied signal's frequency and determining the
piezoelectric device's impedance in response to the swept signal
frequency.
62. The method of claim 58 including detecting a relatively large
signal generated by the piezoelectric device, then determining if
the signal generated by the piezoelectric device varies at a
predetermined rate over a predetermined time, and then concluding
that the object is exerting force on the piezoelectric device.
63. The method of claim 58 including detecting a relatively large
signal generated by the piezoelectric device, then detecting when
the signal decreases relatively rapidly to below a minimum
threshold, and then determining if a constant force is being
applied.
64. A monitoring system comprising:
a piezoelectric device which is subject to a force whose magnitude
is to be monitored and which generates a signal for a change of
force; and
a second device for detecting a relatively large signal generated
by the piezoelectric device, determining if the signal generated by
the piezoelectric device after the detected relatively large signal
varies at a predetermined rate over a predetermined time, and if
so, concluding that an object is still exerting force on the
piezoelectric device.
65. A monitoring system comprising:
a piezoelectric device which is subject to a force whose magnitude
is to be monitored and which generates a signal for a change of
force; and
a second device switching between a first mode and second mode for
detecting a relatively large signal generated by the piezoelectric
device and determining if the signal generated by the piezoelectric
device after the detected relatively large signal decreases over a
brief time interval below some minimum threshold in a first mode,
and if so, determining if a relatively constant force is being
applied in a second mode.
66. A monitoring method comprising:
providing a piezoelectric device which is subject to a force whose
magnitude is to be monitored and which generates a signal for a
change of force;
detecting a relatively large signal generated by the piezoelectric
device;
determining if the signal generated by the piezoelectric device
after the detected relatively large signal varies at a
predetermined rate over a predetermined time; and
if so, concluding that an object is still exerting force on the
piezoelectric device.
67. A monitoring method comprising:
providing a piezoelectric device which is subject to a force whose
magnitude is to be monitored and which generates a signal for a
change of force;
detecting a relatively large signal generated by the piezoelectric
device;
determining if the signal generated by the piezoelectric device
after the detected relatively large signal decreases over a brief
time interval below some minimum threshold; and
if so, determining if a relatively constant force is being applied.
Description
BACKGROUND AND SUMMARY OF THE INVENTION
Piezoelectric materials are well know for their ability to generate
an electrical output when they are placed under mechanical stress.
The frequency and magnitude of an electric signal developed across
such a piezoelectric material are directly proportional to the
frequency and magnitude of a force applied to the material. There
are characteristics of the piezoelectric material's mechanical
system that are analogous to electrical systems. For example, the
material's response to a mechanical force attempting to bend it is
related to the mechanical force in much the same way an impedance
is related to the electrical potential across the impedance.
Piezoelectric materials can be characterized by their inductive,
capacitive and resistive properties. There are numerous
applications of piezoelectric materials' ability to generate
electrical signals which vary with applied force. When the applied
force is not changing, there is no output from the piezoelectric
material.
Piezoelectric materials also undergo mechanical motion when
electrical potentials are applied across them. There are several
applications that make use of this characteristic for the purpose
of changing electrical energy into mechanical energy. A
piezoelectric material's ability to react to electrical energy can
be used to detect the amount of an applied mechanical force. A
piezoelectric device is coupled to a function generator capable of
generating, for example, a sine wave of a desired frequency and
amplitude. If no external force is applied to the piezoelectric
material, the material will deflect back and forth at the same
frequency and relative magnitude to the applied signal. If an
external mechanical force is applied to the material when the
electrical signal is applied, the mechanical force will tend to
inhibit the electrically induced motion of the piezoelectric
material. This resistance to motion will be reflected back to the
electrical system as an impedance to current flow and the applied
electrical energy not manifested as motion is dissipated as heat in
the piezoelectric material. The source subsystem can be constructed
in such a way as to detect this extra impedance, for example, by
monitoring changes in current flow, or, in a constant current
system, by changes in the voltage across the piezoelectric element.
Either way, the force applied to the piezoelectric material is
reflected back to the source of the applied electrical
potential.
According to one aspect of the invention, a monitoring method
comprises providing a first, piezoelectric device which is subject
to a force to be monitored, determining the first device's output,
and correlating the first device's output with the force.
Illustratively according to this aspect of the invention,
determining the first device's output comprises providing
electrical signals for exciting the first device and correlating
the first device's output with the force comprises determining the
first device's response to the electrical signals .
Illustratively according to this aspect of the invention,
determining the first device's output and correlating the first
device's output with the force comprise sweeping the signal
frequency and determining the first device's impedance in response
to the swept signal frequency.
Further illustratively according to this aspect of the invention,
determining the first device's output comprises providing a switch
having a first state corresponding to a dynamic mode of operation
of the first device when the force applied to the first device is
being substantially modulated and a second state corresponding to a
static mode of operation when the force applied to the first device
is not being substantially modulated.
Illustratively according to this aspect of the invention, switching
of the switch to the second state causes a signal to be applied to
the first device and causes the impedance of the first device to
the applied signal to be determined.
Further illustratively according to this aspect of the invention,
determining the first device's output and correlating the first
device's output with the force comprise providing electrical
signals for exciting the first device and determining the impedance
of the first device to the electrical signals.
Additionally illustratively according to this aspect of the
invention, providing electrical signals for exciting the first
device and determining the impedance of the first device to the
electrical signals comprise sweeping the signal frequency and
determining the first device's impedance in response to the swept
signal frequency.
Illustratively according to this aspect of the invention,
determining the first device's output and correlating the first
device's output with the force comprise determining from the
impedance the force on the first device.
Illustratively according to this aspect of the invention, the
method further comprises determining from the determined force
whether an object is on the first device.
Additionally illustratively according to this aspect of the
invention, providing a first device comprises providing a plurality
of said first devices coupled together in an array.
Illustratively according to this aspect of the invention, providing
a first, piezoelectric device comprises providing a flexible
piezoelectric film.
Further illustratively according to this aspect of the invention,
providing a flexible piezoelectric film comprises providing an
array of said first devices.
Illustratively according to this aspect of the invention, providing
a first device comprises providing a coaxial cable including a
center conductor, a shield, and between the center conductor and
shield, a piezoelectric material.
Further illustratively according to this aspect of the invention,
providing a first device comprises providing a piezoelectric
ceramic transducer.
Additionally illustratively according to this aspect of the
invention, providing a plurality of said first devices coupled
together in an array further comprises providing a third device for
holding the plurality of first devices in the array on a detection
surface.
Illustratively according to this aspect of the invention, providing
a piezoelectric transducer comprises custom fabricating a
piezoelectric transducer to a particular size, shape, and
electrical properties.
Illustratively according to this aspect of the invention, the
method further comprises orienting the first device at a strategic
load-bearing point.
Further illustratively according to this aspect of the invention,
providing a first device comprises providing a plurality of first
devices coupled together and oriented at a plurality of strategic
load bearing points so as to permit an object's location to be
determined more accurately.
Additionally illustratively according to this aspect of the
invention, determining the first device's output and correlating
the first device's output with the force comprise detecting a
relatively large signal generated by the first device, then
determining if the signal generated by the first device varies at a
predetermined rate over a predetermined time, and then concluding
that the force is still being applied to the first device.
Further illustratively according to this aspect of the invention,
determining the first device's output and correlating the first
device's output with the force comprise detecting a relatively
large signal generated by the first device, then detecting when the
signal abruptly decreases below some minimum threshold, and then
determining if a constant force is being applied.
According to another aspect of the invention, a monitoring system
comprises a first, piezoelectric device which is subject to a force
to be monitored, and a second device for determining the first
device's output and for correlating the first device's output with
the force.
Illustratively according to this aspect of the invention, the
second device comprises a second device for providing electrical
signals for exciting the first device and for determining the first
device's response to the electrical signals.
Additionally illustratively according to this aspect of the
invention, the second device comprises a second device for sweeping
the signal frequency and determining the first device's impedance
in response to the swept signal frequency.
Additionally illustratively according to this aspect of the
invention, the second device comprises a switch for controlling a
mode of operation of the second device for determining the first
device's output and for correlating the first device's output with
the force, the switch having a first state corresponding to a
dynamic mode of operation when the force applied to the first
device is being substantially modulated and a second state
corresponding to a static mode of operation when the force applied
to the first device is not being substantially modulated.
Further illustratively according to this aspect of the invention,
the second device comprises a signal generator and an impedance
determining circuit, switching of the switch to the second state
causing a signal to be applied by the signal generator to the first
device and causing the second device to determine the impedance of
the first device to the applied signal.
Further illustratively according to this aspect of the invention,
the second device comprises a second device for sweeping the signal
frequency and determining the first device's impedance in response
to the swept signal frequency.
Illustratively according to this aspect of the invention, the
second device comprises a second device for determining from the
impedance the force applied to the first device.
Additionally illustratively according to this aspect of the
invention, the second device comprises a second device for
determining from the determined force whether an object is on the
first device.
Illustratively according to this aspect of the invention, the
apparatus comprises a plurality of said first devices coupled
together in an array so as to permit the object's location to be
determined more accurately.
Additionally illustratively according to this aspect of the
invention, the first device comprises a flexible piezoelectric
film.
Illustratively according to this aspect of the invention, the
flexible piezoelectric film comprises an array of said first
devices.
Additionally illustratively according to this aspect of the
invention, the first device comprises a coaxial cable including a
center conductor, a shield, and between the center conductor and
shield, a piezoelectric material.
Illustratively according to this aspect of the invention, the first
device comprises a piezoelectric ceramic transducer.
Further illustratively according to this aspect of the invention,
the first devices are formed into an array, the apparatus further
comprising a third device for holding the plurality of first
devices in the array on a detection surface.
Additionally illustratively according to this aspect of the
invention, the piezoelectric transducer is custom fabricated to a
particular size, shape, and electrical properties.
Illustratively according to this aspect of the invention, the first
device is oriented at a strategic load-bearing point.
Illustratively according to this aspect of the invention, the
apparatus comprises a plurality of first devices coupled together
and oriented at a plurality of strategic load bearing points so as
to permit an object's location to be determined more
accurately.
Illustratively according to this aspect of the invention, the
second device comprises a second device for detecting a relatively
large signal generated by the first device, placing the second
device in a second state when the relatively large signal from the
first device is detected, determining if the signal generated by
the first device then varies at a predetermined rate over a
predetermined time, and then concluding that an object is still
reposing on the first device.
Illustratively according to this aspect of the invention, the
second device comprises a second device for detecting a relatively
large signal generated by the first device, placing the second
device in a second state when the signal abruptly decreases below
some minimum threshold, immediately switching to the second mode,
and determining if a constant force is being applied.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention may best be understood by referring to the following
detailed description and accompanying drawings which illustrate the
invention. In the drawings:
FIG. 1 illustrates an electrical equivalent circuit of a
piezoelectric device which assists in understanding the
invention;
FIG. 2 illustrates frequency response of the system illustrated in
FIG. 1 as an applied electrical signal's frequency is swept from
1000 Hz to 12 MHZ; and,
FIG. 3 illustrates a partly fragmentary perspective, partly block
diagram of a system constructed according to the invention.
DETAILED DESCRIPTIONS OF ILLUSTRATIVE EMBODIMENTS
Referring now to FIG. 1, a resistance 20 is coupled across a signal
source 22 in parallel with a piezoelectric transducer 24 under
test. Resistance 20 illustratively has a value of 50 .OMEGA.. The
input DC series resistance of the terminals of the piezoelectric
transducer 24 is modeled by a resistance 26. Resistance 26
illustratively has a value of 1 ohm. The input termination
capacitance of the terminals of the piezoelectric transducer 24 is
modeled by a capacitance 28. This capacitance has a value of, for
example, 0.15 .mu.F. The reactance to changing current flow through
the piezoelectric transducer 24 is modeled by two series
inductances, 30 and 32 and a shunt capacitance 34. Capacitance 34
illustratively has a value of 0.15 .mu.F. An internal distributed
resistance of the piezoelectric transducer 24 is modeled by a
resistance 38. Resistance 38 illustratively has a value of 10
.OMEGA.. The output DC termination resistance of the terminals of
the piezoelectric transducer 24 is modeled by a resistance 40.
Resistance 40 illustratively has a value of 2.7 K .OMEGA.. The
output termination capacitance of the terminals of the
piezoelectric transducer 24 is modeled by a capacitance 42 which
also illustratively has a value of 0.15 .mu.F.
The effects of external forces upon piezoelectric transducer 24 can
be appreciated from an analysis of this model. Changing only the
values of the inductances 30 and 32 and keeping all other component
values constant will vary the amplitude of the voltage across
resistance 40. Changing the frequency of the applied electrical
signal causes a change in the transducer 24's characteristic
impedance. Changes in the impedance of the system can be correlated
with changes in its mechanical analog. FIG. 2 illustrates the
transducer 24's frequency responses in the frequency range 1
KHz.ltoreq.f.ltoreq.12 MHz. Each trace is the result of a sweep for
a different value of inductances 30, 32 in the model. Results for
inductance values from 1 to 10 .mu.H in 1 .mu.H steps are
included.
As FIG. 2 illustrates, the frequency response changes with the
change in the inductance. Also, as expected, the frequency of peak
magnitude changes with changes in the inductance. In practice,
these characteristics can be used for detection of a static applied
force on a piezoelectric transducer 24 resulting in a sensor device
much like a load beam. By connecting a controllable voltage source
to piezoelectric transducer 24, and sweeping the signal frequency,
the piezoelectric transducer 24's reactance to the signal can be
determined, and from the reactance the amount of pressure that is
being applied to the piezoelectric transducer 24. Testing pressure
can be applied in the form of gas pressure, water pressure, other
fluid pressure, or the mass of an object such as, for example, a
patient lying still in a hospital bed. Sensors can be constructed
using, for example, sheet piezoelectric transducers 24, coaxial
cable piezoelectric transducers 24, or ceramic piezoelectric
transducers 24.
Devices incorporating piezoelectric transducers 24 can be used as
bed exit or patient monitoring systems on, for example, hospital
beds. Two different modes of operation can be realized. In a first,
dynamic mode of operation, motion of the object, for example, a
patient in a hospital bed, modulates the force on the piezoelectric
transducer 24, causing the piezoelectric transducer to generate an
electric signal. Filtering and signal detection algorithms permit
the presence of the object and, where a plurality of piezoelectric
transducers 24 are used, the location of the object, to be
determined accurately. A limitation of this mode of operation is
that when the object is not modulating the applied force, that is,
not moving, the signal from the piezoelectric transducer 24
practically disappears. Under these circumstances, operation in a
second, static mode is indicated. In the situation where the object
is still on the sensor but not moving, the detection system
switches to the static mode of operation. A signal is applied to
the piezoelectric transducer and the characteristic impedance is
measured. As described before, this characteristic impedance is
used to determine the pressure being (or not being) applied to the
sensor by the object (or lack of object). A determination is then
made if the object is still on the bed surface or not.
Other algorithms can be applied to increase the reliability of the
subject location method and apparatus. As the object moves toward
the edge of the bed, a relatively large signal is generated by the
piezoelectric transducer 24. This relatively large signal puts the
detection system on notice or arms it. If the signal then varies,
for example, diminishes, at a predetermined rate over a
predetermined time, the detection system algorithm concludes that
the object is still on the surface of the bed and there is no exit.
If however, the signal created by the object's movement toward the
edge of the bed abruptly changes to zero or below some minimum
threshold, the system algorithm immediately switches to the
above-described static mode to determine if a constant pressure is
being applied or if, instead, the bed is empty. This reduces the
number of false alarms which would otherwise result from the object
rolling over in bed and then lying still.
Sensitivities to mechanical motion are dependent upon mechanical
sensitivities of the piezoelectric transducers 24, for example,
sheets, cables or ceramic transducers, and the material used.
Ceramic transducers 24 are manufactured by many suppliers who
custom fabricate different sizes, shapes, electrical and physical
characteristics. An array of such ceramic piezoelectric transducers
24 arranged across the detection surface can be constructed using a
pad or other mechanism for holding the piezoelectric transducers 24
in place. The piezoelectric transducers 24 can also be placed at
strategic load bearing points, similar to a traditional load cell
application in a weight measurement device. Piezoelectric film
transducers 24 are flexible and relatively inexpensive. Again, many
suppliers custom fabricate shapes, thicknesses and impregnation of
different types of materials. A sheet of such material or an array
of isolated sensors made from a polymer impregnated sensor can be
fabricated into a suitable detection system. A polymer coaxial
cable manufactured by AMP. (Part nos. 0-1001697-0 and 0-1002399-0)
is a traditional coaxial cable, but the electrical insulator
between the center conductor and the shield is a piezoelectric
polymer. The shield provides a high level of electrostatic
isolation, and its configuration makes installing standard BNC
connectors relatively straightforward. However, its use in static
mode applications is inhibited due to the already physical
containment of the piezoelectric material. Its construction
incorporating the shield and center conductor tends to reduce the
effects of external forces applied to the piezoelectric polymer
insulator, making static mode detection a less than optimal
strategy. However, as a component of a dynamic mode detector, this
material is very effective.
FIG. 3 illustrates a system incorporating various ones of the above
described features into a bed exit or patient monitoring system 50
for, for example, a hospital bed 52. The system 50 includes an
array 54 of piezoelectric transducers 56 which may, for example, be
separate areas of a piezoelectric film of the type described above,
electrically isolated from each other and from their surroundings,
or ceramic piezoelectric transducers of the type described above,
mounted in, for example, a textile or resin retainer 58 which
provides electrical isolation of the various transducers 56 from
each other and from their surroundings. The separate transducers 56
are each coupled to one terminal of respective single pole, double
throw switches . . . 60-c, . . . 60-d, . . . 60e, . . . 60-f, . . .
and so on. Switches 60 may be hardware switches or electronic solid
state switches, but may also be executed in software or firmware
in, for example, a personal computer (PC) 62 which controls
switches 60 according to an algorithm of the type described above
which PC 62 is programmed to execute. One throw of each switch 60
is coupled to, for example, an analog-to-digital (A/D) input/output
(I/O) port of PC 62 to receive the output signals from the
respective transducers 56 when the system 50 is operating in the
dynamic mode described above. These output signals from the
transducers 56 may, for example, be time-division multiplexed into
the PC 62. The other throw of each switch 60 is coupled to a signal
generator 64. While separate signal generators . . . 64-c, . . .
64-d, . . . 64-e, . . . 64-f, . . . and so on, are illustrated for
each respective switch . . . 60-c, . . . 60-d, . . . 60-e, . . .
60-f, . . . and so on, fewer signal generators can be employed. For
example, it is contemplated that a signal generator 64 may be
implemented as an output device of PC 62, or PC 62 may incorporate
a digital waveform synthesis routine, with the synthesized waveform
being appropriately processed, for example, amplified by an output
device of PC 62, or directly supplied to the switches . . . 60-c, .
. . 60-d, . . . 60-e, . . . 60-f, . . . and so on. The signal
generator(s) 64 will ordinarily be under the control of the PC 62,
and will only be activated when it (they) is (are) required, such
as, for example, when the system 50 is operating in the static mode
described above. If the signal generator(s) 64 is (are) implemented
by synthesizing it (them), the signal generator 64 output(s) can
also be time-division multiplexed among the transducers 56 that
require excitation in the static mode of operation. And, as noted,
whatever transducer(s) 56 is (are) excited by the signal generator
64 will need to be monitored, illustratively by the PC 62's A/D I/O
port, to determine, for example, the voltage(s) across the
transducer(s) 56 in response to excitation. From this (these)
measure(s), the impedance(s) of the transducer(s) 56 being excited
can be determined, and from the impedance(s), the force(s) on the
transducer(s) 56 being excited.
* * * * *